。
She's still no more than a child.
Literal
[topic-は] still child [no-more-than-にすぎない].
~にすぎない ('no more than X, merely X') is a fixed expression of restriction — it minimizes the subject's status, claiming X is all there is. Combined with まだ ('still') it doubly emphasizes that despite whatever expectations might have been raised, the person is still just a child. 子供にすぎない is the bald 'just a child' frame, which can soften criticism (excusing a young person's behavior) or make a point about immaturity in older subjects. Reads slightly defensive — like someone explaining away or excusing the woman's behavior on the grounds of her youth.